The present invention relates to a control system for controlling shock occurring upon the engagement of a torque transmitting frictional element in an automatic transmission.
Commonly, in an automatic vehicle having an automatic transmission, a so-called creep phenomenon occurs after the vehicle has come to a halt with the transmission selector placed in the D position. The creep occurs because a torque-transmitting frictional element remains engaged in order to maintain a torque delivery path for ease of subsequent start-up of the vehicle. Engine torque multiplied by the torque converter is thus supplied via the torque delivery path to the driving wheels of the vehicle even when the engine is idling. In order to solve this problem, it has been proposed in JP-A-58-128552 that the frictional element be released when the vehicle is at a standstill. In this case, the hydraulic fluid pressure supplied to the frictional element must be strictly controlled when the predetermined frictional element is engaged to start the vehicle moving from a standstill. A control system for controlling hydraulic fluid pressure for a frictional element is disclosed in JP-A-60-220247.
According to JP-A-60-220247, the rotational speed of a turbine shaft or of a transmission input shaft is detected, and an amount of slip of the frictional element is hydraulically controlled in such a manner that the detected rotational speed approaches a target value, thereby engaging the frictional element without substantial shock. However, this control system causes a hunting phenomenon to occur which increases as the deviation of the detected rotational speed N.sub.T increases relative to the target value N.sub.T0. Thus, the transmission output torque curve peaks immediately after the transmission selector is moved from the N (neutral) range to the D (drive) range as shown in FIG. 9. The occurrence of this peak in the torque causes substantial shock to occur.
An object of the present invention is to provide an electro-hydraulic control system for controlling the pressure of hydraulic fluid predetermined supplied to a predetermined frictional element wherein hunting is suppressed to a satisfactorily low level even if a deviation of a detected input rotational speed from a target rotational speed is large.